1. Field of the Invention
The present invention relates generally to a molding apparatus such as an injection molding apparatus, more specifically to a mold clamping unit employed in such a molding apparatus.
2. Description of the Related Art
A typical conventional mold clamping unit employed in an injection molding apparatus, for example, for foam molding comprises a mold clamping cylinder in which a ram is directly mounted on a movable mold plate of the mold. In such a mold clamping unit, opening and closing of the mold is conducted by bringing the movable mold plate in contact with the fixed mold plate and withdrawing the movable mold plate from the fixed mold plate by extension and contraction of the mold clamping cylinder.
When foam molding is conducted, the cavity of the closed mold is filled with plasticized resin, then the mold clamping cylinder is contracted and the movable mold plate is retracted through a very small distance to a half-open position. As a result, the cavity is expanded. After waiting for the plasticized resin to foam in this state, the mold clamping cylinder is further contracted, so that the movable mold plate is retracted to a final fully open position, and the molding is then removed from the mold.
However, the problem arising when the above-described conventional mold clamping unit is used is associated with the appearance of defects such as a variation in the thickness of a molding. This is because one mold clamping cylinder is used in the conventional mold clamping unit for moving the movable mold plate from the closed position to the half-open position and then to the fully open position. Thus, the mold clamping cylinder for moving the movable mold plate between the closed position and fully open position has a large capacity and, therefore, requires a large amount of working fluid for driving the mold clamping cylinder. As a result, when the movable mold plate is retracted through a very small distance from the closed position to the half-open position, a variation in the distance traveled by the movable mold plate can easily occur, leading to a variation in the thickness of a molding.
Another problem is that when a large-capacity mold clamping cylinder is used, the response during the retraction of the movable mold plate from the closed position to the half-open position is poor and high-speed operation is difficult to conduct. This is because a comparatively large amount of working fluid is required for retracting the movable mold plate from the closed position to the half-open position with the large-capacity mold clamping cylinder. A poor response of the actuation of the movable mold plate to the half-open position and a low operation speed also adversely affect the quality of foamed moldings.
Accordingly, a main object of the present invention is to provide a mold clamping unit capable of accurately moving the movable mold plate from the closed position to the half-open position.
Another object of the present invention is to provide a mold clamping unit capable of moving the movable mold plate from the closed position to the half-open position with good response and a high speed.
In order to attain the above-described objects, the present invention provides a mold clamping unit which is used in a molding apparatus and serves to open and close a mold having a movable mold plate and a fixed mold plate, this mold clamping unit comprising a mold clamping cylinder driven by supply or discharge of a working fluid and moving the movable mold plate between a fully open position located away from the fixed mold plate by the predetermined distance and a closed position in which the movable mold plate is in contact with the fixed plate, mold opening means for applying a mold opening force of a predetermined magnitude to the movable mold plate in the direction from the closed position toward the fully open position, thereby moving the movable mold plate from the closed position to the predetermined half-open position located between the closed position and the fully open position, switching means for switching the mold clamping force generated by the mold clamping cylinder between a first mold clamping force which is larger than the mold opening force and a second mold clamping force which is smaller than the mold opening force by switching the supply pressure of the working fluid supplied to the mold clamping cylinder, and control means for controlling the mold opening means and the switching means.
With this arrangement, a special mold opening means is separately provided, in addition to the mold clamping cylinder, for moving the movable mold plate from the closed position to the half-open position. Therefore, the movable mold plate can be accurately moved to the half-open position. Furthermore, such a movement can be conducted with a high speed and good response.
Various types of control of mold opening means and switching means can be considered, but it is preferred that the switching means be controlled so that the mold clamping force becomes the first mold clamping force when the movable mold plate is moved to the closed position. In such a case, even if the mold opening means is activated, the mold can be closed because the first clamping force is larger than the mold opening force generated by the mold opening means. Furthermore, if the switching means is controlled so that the mold clamping force becomes the second mold clamping force when the movable mold plate is moved from the mold clamping state to the half-open position, the movement of the movable mold plate from the closed position to the half-open position can be initiated within a short period of time because the mold opening force is larger than the second mold clamping force. Thus, the response of mold opening operation to the half-open state is improved. In addition, since the movable mold plate is in a state in which it is pushed in the mold clamping direction by a mold clamping cylinder, the movable mold plate that was retracted to the half-open position by the mold opening means can also be prevented from further moving in the direction toward the fully opened position under an inertia force.
Furthermore, for example, when foam molding is conducted in which the movable mold plate is retracted from the closed position to the half-open position so that the plasticized resin inside the cavity of the mold is foamed, the plasticized resin placed into the cavity of the mold in the half-open state will be partially separated from the cavity surface because of volume shrinkage caused by decrease in temperature. As a result, the surface shape of a molding sometimes does not accurately match that of the cavity and defects are formed in the molding.
Accordingly, the control means may also control the switching means so that the mold clamping force becomes the first mold clamping force for the purpose of moving the movable mold plate again toward the closed position after it was moved to the half-open position. As a result, the resin in a semihardened state is again compressed in the mold and, therefore, a molding can be prevented from being strained.
When the mold clamping unit further comprises a mounting stand on which the fixed mold plate is mounted and a mounting plate on which the movable mold plate is mounted, the mold opening means may be that comprising an extension and contraction mechanism which is disposed between the mounting stand and mounting plate and can extend and contract in the mold clamping direction, a drive source for driving the extension and contraction mechanism, and stop means for terminating the extension and contraction operation of the extraction-contraction mechanism once the movable mold plate has been moved from the closed position to the half-open position.
In this case, it is preferred that a hydraulic cylinder be used as the extension and contraction mechanism, that a hydraulic pump supplying working fluid to the hydraulic cylinder be used as the drive source, and that the end portion of a cylinder tube at the rod side thereof, which is brought in contact with the piston when the hydraulic cylinder is extended, be used as the stop means.
Providing, between the extension and contraction mechanism and the mounting stand, a position adjusting tool that adjusts the position of the extension and contraction mechanism in the mold clamping direction can also be effective. This is because even when the mold is replaced by another mold of different size, the position of the extension and contraction mechanism can be adjusted by the position adjusting tool.
A preferable position adjusting tool is that comprising a first block having a first surface and a second surface opposite to the first surface and a second block having a third surface that is slidably in contact with the second surface of the first block and a fourth surface opposite to the third surface and has a configuration, with respect to the first and second blocks, such that the spacing between the first surface and the fourth surface can be adjusted by the mutual wedge effect of the first block and second block.
The present invention in another aspect thereof provides an injection molding apparatus comprising the above-described mold clamping unit, a mold, and an injector for injecting a plasticized resin into the mold. Because the mold clamping unit has the above-described operation effect, the injection molding apparatus can make moldings have thickness uniform therebetween, and can produce high-quality moldings.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.